1. Field
The field is pad printing, and in particular rotary transfer pad printing.
2. Prior Art—FIGS. 1 through 3
In the past, rotary pad printing has been used to decorate objects by printing images or text thereon. FIGS. 1 and 2 show top and side views, respectively, of a prior-art rotary pad printing apparatus. In its simplest form, the rotary pad printing apparatus comprises a rotary cliché 100, i.e. a rigid, cylindrical surface etched using well known photolithographic methods (not shown) about 0.03 mm deep with an image to be printed, a cylindrical wheel 107 comprising a resilient or deformable layer 105 that encircles a rigid shaft 106, and an ink source 110 arranged to supply ink to the etched regions of the cliché.
For exemplary proposes, assume that the numbers 1 through 5 are etched in cliché 100. Although numbers are shown here, the image can comprise text, graphics, and even photographic information. Next, cliché 100 is placed into the printing apparatus adjacent wheel 107 as shown in FIGS. 1 and 2, and ink source 110 is filled with ink.
When printing commences, cliché 100 and wheel 107 are driven to rotate against one-another by a known mechanism (not shown). Directions of motion are indicated by the arrows in FIG. 2. A series of objects 115 to be decorated or printed on are arranged to move upward past wheel 107 and in contact with layer 105. As cliché 100 turns, ink from source 110 is doctored into the etched surface of cliché 100 by ink source 110, in well-known fashion.
As cliché 100 turns against wheel 107, image-wise regions of ink 120 are transferred in near-entirety from cliché 100 to wheel 107. As rotation of wheel 107 continues, ink regions 120 are brought into contact with the upper object 115. As ink regions 120 come into contact with object 115, they leave wheel 107 and most of the ink in regions 120 transfers to object 115. Thus the image originally present on cliché 100 is transferred to object 115.
Instead of moving objects 115 with their axes perpendicular to that of wheel 107, objects 115 can have a round or ellipsoid cross-section and can be rotated against wheel 107 with their axes of rotation parallel. This type of transfer motion is shown below.
This prior-art arrangement is suitable only for single-color transfers. If the user wishes to transfer multicolor images, color separations are required and a separate cliché is required for each color. The objects to be decorated must be carefully aligned for subsequent passages through the rotary transfer pad printing apparatus.
Since the image on the cliché is etched, each cliché contains only one image. Changing the image requires etching a new cliché and exchanging the new cliché for the old one. Since this operation is somewhat involved, it is impractical to use the prior-art apparatus to print small runs. In addition, the prior-art apparatus can not change or update each succeeding image.
Since the cliché transfers the etched and inked image to the rotary pad, the length of the image that can be transferred is limited by the circumference of the cliché.
Also the distance between the axis of the rotary pad and the surface supporting the image-receiving object is fixed. The amount of normal force applied at the point of transfer is dependent upon the compressibility and thickness of the rotary pad. This limits the available variation in height of the object to be printed. In many cases, the compressible rubber part of the pad wheel is about 2.5 cm thick. Thus the thickness change on compression from first contact to maximum compression is significantly less than 2.5 cm.